The digital speedometer on 1985-88 cars had a built-in reminder to take the car in for routine services, such as an oil change. It is simply meant as a reminder and is not a strict schedule per se, but it does give you a gentle reminder to keep routine maintenance in the forefront.
To remove the SERVICE reminder, simply press the TRIP and TRIP RESET buttons simultaneously and hold for a few seconds, until you hear 3 beeps. The SERVICE reminder will then go out.
If you have just an S in a circle on the speedometer display, that indicates a replacement speedometer module. More information about removing the S warning can be found here.